1. Field of Invention
The present invention relates generally to protective enclosures for electronics and electronic circuits.
2. Description of the Prior Art
Electronic circuitry assemblies, printed circuits boards, and substrates containing circuitry and electronic components mounted thereon, often require electromagnetic interference (EMI) shields to limit the likelihood of signal interferences from electromagnetic waves, such as those caused by radio-frequency (RF) signals.
It is known that the operation and performance of sensitive electronic components, such as integrated circuits (ICs), can be affected by the presence of interfering electromagnetic signals. Certain electronic components and devices are known to emit electromagnetic signals during their operation. In particular, on a circuit board, components emitting EMI signals can detrimentally affect the performance, reliability and even operability of other electronic components on the same board. Three essential elements must be present for an EMI situation to exist, including: an electrical noise (EMI) source, a coupling path, and a victim receptor. The noise source emission can be either a conducted voltage or current, or an electric or magnetic field propagated through space. It is known that certain equipment and systems can serve as both EMI sources and receptors. A coupling path may exist between signal sources and receptors and can be divided into two basic groups: radiation or field coupling by electromagnetic wave propagation through space or materials (hereinafter known as xe2x80x9cair couplingxe2x80x9d), and coupling via conducted paths through which current can flow (hereinafter known as xe2x80x9cboard couplingxe2x80x9d). Additionally, and herein after, the use of the term xe2x80x9cboard couplingxe2x80x9d includes the transmission of EMI across a circuit as well as electromagnetic wave propagation through the circuit board or substrate material and the term xe2x80x9cair couplingxe2x80x9d includes the transmission of EMI through the air due to electric field and/or magnetic field emanations.
To minimize the presence of interfering signals with sensitive electronic components and the effects of air coupling and board coupling, whether the interfering source is on the same assembly or apart from the receptor device being a sensitive electronic, the use of EMI shielding is often employed.
EMI signals may occur and interfere with electronic components due to sharing of conductors with EMI sources, emanation of electric fields, emanation of magnetic fields and electromagnetic radiation. EMI shielding causes an electromagnetic wave propagating through space to be absorbed or reflected when the wave contacts the shield wall. EMI signals are first reflected off a shield wall usually due to the material of the shield. EMI signals, also known as energy, which are not completely reflected may then be absorbed by the shield wall such that only residual energy is able to emerge from the opposing side of the shield wall. The emergent residual energy is the resulting EMI. Therefore, the effectiveness of the shielding is determined by the shield""s reflectance and absorbance characteristics, which are dependent on the shield material and the shield interface with the substrate and circuit ground.
EMI shields are often installed over or in proximity to sensitive electronic components on a circuit board to inhibit interference from propagating to a receptor or to prevent EMI signals from being emitted by an emitting source. An EMI shield may be of varying in shape and size in relation to the sensitivity of the electronics and the material used in construction of the EMI shield. Generally, EMI shields range in size from one square inch to over two square feet.
It is known that an EMI shield is typically comprised of a metal sheet, a casting, or other conductive material such as a mesh or paint which is formed into a shape in relation to both the components and the space available on the circuit board. An EMI shield is usually precisely placed on a circuit board at a prescribed location and is attempted to be grounded, usually with a circuit ground. The EMI shield is typically installed by securing the shield to the circuit board. Often, compression fittings or screws are used to secure the shield in place. However, in securing the shield, often shield edges move slightly from the prescribed location, thereby affecting shield performance. It is known that the precision placement of the shield in contact with the substrate and a ground is critical to effectively isolate the circuit from interference and effectively ground errant electromagnetic waves.
Problems frequently arise due to movement of the shield during securing and also due to gapping along the interface between the shield edge and circuit board. In particular, the surfaces of the standard boards and shield edges are not uniformly planar and these nonuniformities cause inconsistencies in contacts due to spaces or gaps resulting at the interface between a board and a mounted shield edge. Similar problems also arise when shield edges are improperly positioned by as little as 0.0001 inches from the predetermined location. Unfortunately, these problems usually cause EMI coupling, such as interferences due to common impedance (e.g. board coupling) and electric and magnetic fields (e.g. air coupling), resulting in EMI signals interfering with sensitive components.
Prior art solutions, in attempting to resolve these problems, have maintained the use of a shield edge having a surface which is finished flat, as conventional belief is that a flat edge limits the movement of the edge in relation the edge""s position on the substrate. However, the prior art""s use of a flat edge surface does not overcome problems resulting from movement, gapping or the effects of nonplanar surfaces. A flat edge surface does not substantially prevent EMI effects, and in particular, does not substantially limit EMI effects caused by air coupling. Consequently, the prior art has been less than satisfactory.
In FIG. 1, prior art EMI shields and substrates are depicted. FIGS. 1A through 1C depict shields which are to be secured onto a substrate at precise locations using a securing means, such as a plurality of screws (not pictured). Each of these prior art figures necessitates that the shield edge be precisely aligned with the predetermined location on the substrate, as previously discussed.
FIG. 1A depicts a conductive gasket 10 to be placed on a substrate 20 and a casted EMI shield 30 having a cut-out 40 fitted to receive the gasket 10. The gasket 10 is sandwiched into the cut-out 40 and the resulting shield 30 and gasket 10 are compressed onto the substrate 20 and secured by a securing means. FIG. 1B depicts a gasket 50, which forms in place on the planar contact surface 60 of the shield 65. When compressed into place, the gasket 50 also forms to take the shape of the surface 75 of the substrate 70. The gasket 50 is comprised of a deforming material, which alters its shape when compressed under pressure or at elevated temperature. A machine (not pictured) is typically used to form the gasket 50 in place along the edge 60 of the shield 65. The shield 65 and gasket 50 are then compressed into place on the surface 75 of the substrate 70 and secured by a securing means.
FIG. 1C depicts a non-contacting interface 80 between a substrate 90 and an edge 95 of a shield 99.
Each of the FIGS., 1A through 1C, shows a cross-sectional view when the substrate, the grounding means and the shield are each planar with respect to one another. In situations where these elements are not planar, the interface contact is interrupted or is inconsistent and the EMI reflectivity and absorbtivity of the shield at the interface is generally unsatisfactory. Additionally, each of the prior art depictions is generally ineffective in substantially preventing EMI due to board coupling and air coupling effects, since gaps at interfaces result in substantially reduced shielding performance.
Attempts have been made to improve the contact by using post-fabrication planar edges and boards. Following initial fabrication, edges of the EMI shields are finished flat to remove burrs and imperfections and usually the shield edge is planed flat. Similar steps during or subsequent to manufacture are made to improve board surfaces. These steps add considerable time and expense to the EMI shield and board, and often result in minimal shielding improvement. Further, attempts to improve the contact and performance of the shielding have also included providing additional screws in close relation to one another. The addition of screws to shield-board assembly results in limited improvements since variations in performance of the shield across the shielded area are common due to the variations of torque exerted on each screw. Minor deviations in torque on each screw detrimentally and significantly affects the overall performance of the shield, causing certain shielded areas to have better shielding than other. Further, the addition of screws to a mounted shield has also been used to attempt to reduce air coupling problems resulting from EMI signal propagation. However, these attempts have been generally unsatisfactory for similar reasons.
FIG. 2 is a diagrammatic view of an EMI shield 100, having a planar shield edge 105, mounted and secured to a substrate 110, at an interface 120, with a plurality of screws 130 for securing the shield 100 to the substrate 110.
Consequently, unless each of the screws is secured with approximately the same torque, and unless a planar shield edge is used with a planar circuit board, performance variations across the shield will likely exist and the chance of edge movement in relation to precision placement of the shield thereby increases. Additionally, with the increased number of screws, there is a proportional decrease in the space available for components. These detrimental factors increase costs of manufacture and labor time required to precision mount and adjust shielding devices in an attempt to obtain desired performances.
Accordingly, there exists a need to economically and consistently produce a solution utilizing standard circuit boards and EMI shields which requires minimal post-manufacturing fabrication and provides electrical isolation from electromagnetic interference due to board coupling and air coupling, while reducing the need for securing hardware.
The present invention provides a conductive connectivity between an EMI shield edge and a means capable of performing the function of grounding the shield with an infinite sink along the interface with the shield edge. The means is electrically configured to provide electrical interconnections between a ground sink and shield edges, which are fitted on an intermediary or the substrate, which is in electrical conductivity with the means. In a preferred embodiment, the edges are fitted into the intermediary. Further, and hereinafter, this means is referred to as the grounding means which includes the ability to provide a ground, a ground path, and a grounded intermediary, as well as electrical connectivity and conductivity to each. The grounding means also has the capability to provide connectivity to a ground having an infinite sink.
The present invention provides a simple EMI shielding assembly with a dielectric substrate, having at least one electronic component mounted thereon, in which simply finished angled EMI shield edges are adaptably and non-precisionly fitted and secured onto the substrate and remain in conductive contact with the grounding means, thereby substantially excluding EMI effects of board coupling and air coupling from shielded electronics.
The present invention also provides a simple EMI shielding assembly with a dielectric substrate, having at least one electronic component mounted thereon, in which simple unfinished rough EMI shield edges are adaptably and non-precisionly fitted and secured into an improved substrate having an angled mating groove extending below the substrate surface, in which EMI shield edges remain in conductive contact with the grounding means, thereby substantially excluding EMI effects of board coupling and air coupling from shielded electronics.
The present invention further provides a simple EMI shielding assembly with a dielectric substrate, having at least one electronic component mounted thereon, in which simply finished angled EMI shield edges are adaptably and non-precisionly fitted and secured into an improved substrate having an angled mating groove extending below the substrate surface, in which EMI shield edges remain in conductive contact with the grounding means, thereby substantially excluding EMI effects of board coupling and air coupling from shielded electronics.
Additionally, the present invention provides a method for fabricating an EMI shielding assembly with a dielectric substrate in which simply finished angled EMI shield edges are adaptably and non-precisionly fitted and secured into an improved substrate having an angled mating groove extending below the substrate surface, in which EMI shield edges remain in conductive contact with the grounding means, thereby substantially excluding EMI effects of board coupling and air coupling from shielded electronics.
Further objects and advantages of this invention will become apparent from the detailed description, which follows.
In general, the present invention provides an economical protective shield assembly of a dielectric substrate and an EMI shield, substantially free of the effects of circuit coupling and air coupling while using a reduced set of hardware for securing an EMI shield into place. The use of simply finished angled shield edges adaptably fitted with a substrate having a solder bead ground path, conductive with a grounding means, interacting to solve the problems previously discussed, is unique and has not been heretofore realized. Similarly, the use of simply finished angled or simple unfinished (i.e., rough) angled shield edges adaptably fitted with an improved substrate having an angled mating groove conductive with a grounding means, interacting to solve the problems previously discussed, is unique and has not been heretofore realized.
The invention also provides a simplified solution to the exclusion of EMI effects due to board coupling and air coupling by a fabrication process requiring fewer finish steps than prior art processes. The use of simply finished angled shield edges adaptably and non-precisionly fitted with an improved substrate having an angled mating groove and a ground path conductive with a grounding means, interacting to solve the problems previously discussed, is unique and has not been heretofore realized.
The shield assembly substantially excludes EMI board coupling and air coupling from electronic components, which are shielded by the assembly. The shield assembly also substantially prevents EMI board coupling and air coupling from electronic components emitting EMI signals from within the assembly. The assembly provides a grounding means having a ground along the ground path. The ground path traverses the interface between the shield edge and either the substrate or the intermediary, which is in contact with the substrate, depending on the configuration.
Propagating EMI signals encountering the shield are reflected or absorbed by the grounded EMI shield, which effectively acts as an infinite sink as it is in conductive contact with an infinite sink. The non-reflected energy, also known as absorbed signals, are routed to the sink along each of the shield walls in which the energy is absorbed. Due to the grounded shield (with the ground path), substantially all of the absorbed signals are routed to the ground. Board coupling and air coupling are substantially eliminated as the EMI shield is provided a continuous ground through the intermediary at the interface, and the assembly substantially excludes EMI signal emanating from electric fields, magnetic fields and electro-magnetic radiation.
The assembly is comprised of a dielectric substrate, having at least one component electrically mounted on the surface of the substrate, an EMI shield having contact edges fitted on the substrate, shielding at least one mounted component, and a grounding means electrically grounding the shield. The assembly preferably does not necessitate the precision placement of shield edges with the substrate to the extent of the prior art.
The dielectric substrate provides a mounting base and is functional to provide electric current, via printed circuit patterns, to mounted electronic components. Preferably the substrate is rigid and nearly planar since the securing means is secured into the substrate. The securing means may secure the shield to the substrate in a variety of ways including but not limited to screws being inserted through the back of the substrate into a shield, and/or mounting a pair of shields xe2x80x9cback-to-back,xe2x80x9d one on each side of the substrate. The dielectric substrate may be a printed circuit board (PCB) or a circuit and a base assembly. A location on the substrate, for placing and securing the shield is predetermined, and is referred to herein as the xe2x80x9cshield tracexe2x80x9d.
A grounding means provides electrically conductivity with and along the ground trace. The ground trace is at and along the same predetermined location as the shield trace. The ground trace when conductive with the grounding means shall also be referred to herein as the xe2x80x9cground pathxe2x80x9d. The shield, once attached to the substrate along the ground trace, is electrically grounded to the zero potential of the ground by the ground path through the grounding means. An intermediary is provided along the ground path at designated positions, wherein the intermediary between the interface of the substrate and the shield edge is provided for contact between the shield edge and the ground. Preferably, the intermediary is a material selected from the group of solder, copper or indium.
The EMI shield may first be shaped, by fabrication processes such as casting or forming for instance, and is then positioned at a predetermined location onto the shield trace. The shield is comprised of a material suitable for effectively and substantially reflecting and/or absorbing EMI signals. Edges of the shield are located along the lower portion perimeter of the shield. Edges may be xe2x80x9croughxe2x80x9d (i.e., unfinished) without post-fabrication processes to smooth the angular sides of the edges. Edges may also be finished, by using an edging means, which is capable of smoothing the roughness from the edge sides and surfaces. Preferably, the edge is simply and angularly finished.
The contact surface of each edge fittably contacts the substrate, or the intermediary, when the shield is mounted and secured onto the substrate to form the assembly. The contact surface of each edge is fitted on the substrate, along the shield trace, in contact with the intermediary. At the point of contact between the substrate and the shield, through the intermediary along the ground path, an interface is formed.
In accordance with one aspect of the present invention, the dielectric substrate comprises a solder bead on the surface of the substrate, above the horizontal axis of the substrate surface, which is functional as a ground path and is electrically conductive with the grounding means, and the EMI shield comprises angled finished edges adapted to be fitted into the solder ground path, such that the assembly is substantially free of the effects of board coupling and air coupling.
In accordance with another aspect of the present invention, the dielectric substrate comprises an angled mating groove for each unfinished shield edge, in which each groove has angled walls and is positioned below the horizontal axis of the substrate surface and has a ground path which is in electrical conductivity with the grounding means, such that the assembly is substantially free of the effects of board coupling and air coupling.
In accordance with another aspect of the present invention, the dielectric substrate comprises an angled mating groove for each finished angled shield edge, in which each groove has angled walls and is positioned below the horizontal axis of the substrate surface, and the EMI shield comprises cooperatively-angled finished edges adaptably fittable into the groove which has a ground path in electrical conductivity with the grounding means, such that the assembly is substantially free of the effects of board coupling and air coupling.
In accordance with a further aspect of the present invention, a method for a simplified fabrication of an assembly, requiring a reduced set of hardware for securing the shield, in which the assembly is substantially free of the effects of board coupling and air coupling, is provided for.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with accompanying drawings.